Correctly ignore "self" for border radius clipping when painting the background phase or fragments.

This was broken in crrev.com/653303003.

There is no test for the latter, but that code is not live yet. Punting to 
mstensho@opera.com.

BUG=425030

Review URL: https://codereview.chromium.org/694753004

git-svn-id: svn://svn.chromium.org/blink/trunk@184831 bbb929c8-8fbe-4397-9dbb-9b2b20218538